This proposal addresses a critical need in the medical ICU: quantifying regional pulmonary compliance. At present, Acute Lung Injury (ALI), including Acute Respiratory Distress Syndrome (ARDS), has an incidence of almost 100 cases per 100,000 person years in the USA, with hospital stays averaging more than 2 weeks, and with mortality between 20-40%. Besides pharmacologic treatment of underlying disease and cardiovascular compromise, ventilating the critically ill patient represents special challenges. Based on phenomenologic measures of outcomes such as O2 and CO2 transport, a number of empirical guidelines exist specific to tailored tidal volume, frequency, and positive end expirator pressure (PEEP). The target here is a ventilatory strategy with a window that promotes adequate gas exchange without further damage to the lung through baro- or volutrauma. While there are some reports of reducing the mortality rate, it still remains unacceptably high. What is lacking is a technology by which local regions of the lung can be assessed along the mechanical spectrum from collapse (and associated right-to-left shunt) to overexpansion (and associated further ventilatory damage). The specific measure needed is local lung compliance, and the ability to follow lung compliance changes with varying levels of ventilator parameters, particularly tidal volume and PEEP. It is essential that this be possible in the local setting of te ICU, with data available essentially in real time, without requiring moving patients to e.g. separate CT suites for lung patency assessment. To meet this need, we propose a portable Magnetic Resonance Lung Density Monitor (MR-LDM) to be used at the ICU bedside. The MR-LDM is a low field permanent magnet with a field of 0.0085T. A target 20cc region of field homogeneity (3.4cm DSV) is produced ~8cm from the surface of the MR-LDM so that this homogeneous field region extends into the lung when the device is placed on the chest surface. At the low magnetic field we use, there are no magnetic susceptibility artifacts and the measured signal intensity from the lung is directly proportional to local proton density, thus reflecting local tissue density. Our specific aims are to demonstrate that pulmonary compliance can be measured with this device. Compliance measurements made with the MR-LDM will be compared with those obtained with traditional MRI at 1.5T, from which measurements of lung density as a function of volume will be measured. Excised pig lungs, both normal and surfactant washed to induce patchy atelectasis, will be used to demonstrate the MR-LDM can detect regions of pathology independently measured with CT.